Composite microwave window and waveguide transform

ABSTRACT

A section of relatively high impedance waveguide is connected via abrupt waveguide transitions at opposite ends thereof to second and third waveguides having substantially lower characteristic impedance. A gas-tight wave permeable window member is hermetically sealed across the center section of high impedance guide. The second waveguide has a substantially lower characteristic impedance than the third waveguide and the window member is displaced in the high impedance guide toward the low impedance guide to form a relatively broad band impedance match, thereby forming the composite window and waveguide transformer structure.

United States Patent [1 1 Butwell 11] 3,753,171 1 Aug. 14, 1973 COMPOSITE MICROWAVE WINDOW AND WAVEGUIDE TRANSFORM [75] Inventor: Robert J. Butwell, San Jose, Calif. [73] Assignee: Varian Associates, Palo Alto, Calif. [22] Filed: Apr. 5, 1971 [21] Appl. No.: 131,209

[52] U.S. C1 333/98 1, 333/33 [51] Int. Cl. H01p 1/08, l-lOlp 1/00 [58] Field of Search 333/98 P, 35, 33, 333/98 R [56] References Cited UNITED STATES PATENTS 3,309,558 3/1967 Heil 333/98 P 2,698,421 12/1954 Kline et al 333/98 P X 2,958,834 11/1960 Symons et a1. 333/98 P 3,221,206 11/1965 Miller 333/98 P X 3,221,278 11/1965 Winslow 333/98 R 3,278,865 10/1966 Forrer 333/98 R OTHER PUBLICATIONS Hudson, A. C., Circular Waveguide Chart," Electronics 10-1954, pp. 194

Bethe, H. A., Theory of Diffraction by Small Holes," Physical Review vol. 66, 1944, pp. 177-178 Collins, G. 13., "Microwave Magnetrons," McGraw Hill, 1948, PP. 488

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. I-I. Punter Attomey--Stanley Z. Cole ABSTRACT 5 Claims, Drawing Figures Patented Aug. 14, 1973 FREQUENCY GHz INVENTOR.

ROBERT J. BUTWELL BY QQL ATT EY 1. COMPOSITE MICROWAVE WINDOW AND WAVEGUIDE TRANSFORM 1 DESCRIPTION OF THE PRIORART dow has the advantagethat it is capable of passing relatively high microwave power levels, as of 4MW peak at Sband, and relatively high average powers, as of 8 KW average, over a relatively wide band of frequencies, asof to 30% with a VSWR less than 1.1.

This prior art window is typically usedas the output window of a high powerpulsedrriicrowave tube, such as a klystron, traveling wave tube or magnetron. However, in such applications, thewaveguide feeding outwardly of the microwave tube generally has a reduced height compared to normal waveguide to accommodate magnetic beam focusing structure and the like typically surroundingthe tube. Also, the output waveguide normally has a reduced height in order to have a lower characteristic impedance for providing an improved waveguide match to the outputload impedance of the tube.

Therefore, in the prior art; a multiplestep waveguide transformer section was provided between the output of the microwave tube andthe microwave window,

such that the input side of the microwave window could be connected to standard height waveguide.

The problem with this arrangement is that such a waveguide transformer is generally relatively costly and it would be desirable to eliminate the separate transformer if possible.

SUMMARY OF THE PRESENT INVENTION The principal object .of the present invention is the provision of an improvedcomposite microwave window and waveguide transformer.

One feature of the present invention is the provision of a composite microwave window and waveguide transformer wherein afirst section of waveguide, which band impedance match to form a composite microwave window and transformer.

In another feature of the present invention, the first waveguide is of circular cross section and the second and third connecting waveguides are of rectangular cross section andthe abruptwaveguide transitions include walls at opposite ends of the first waveguide section with rectangular apertures for providing wave en LII ergy communication between the second and third and first waveguide sections.

In another feature of the present invention, thelow impedance rectangular'connecting waveguide has a heightof approximately /5 of itswidth and the third waveguide is rectangular having a height which is approximately V: of its width.

Other features and advantages of the present inven tion will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWlNGS- 1 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1,-there is shown the prior art microwave window 1. Thewindow structure 1 includes a first section of hollow cylindrical waveguide 2 approximately an integral number of IIQWZVCIBHEIIIS long atthe center frequencyof the passband of the window 1. The cylindrical section of waveguide 2 is closed at opposite ends by centrally aperture-d end walls 3 and 4. Rectangular waveguides Sand 6 of substantially lower height waveguidethan the center cylindrical section 2 are connected to the center section; 2 via therectangular apertures 7 and 8 in opposite end walls 3and 4.The apertures 7 and 8 are of the same dimensions as the inside dimensions of the rectangular waveguidesSand 6, respectively. The centrally apertured end walls 3and4 provide abrupt waveguide transitions from the waveguides 5 and 6 to the central waveguide section 2.

A disc shaped wave permeable window member 9, as of alumina ceramic, is heremetically sealed, as by brazing, transversly across the central waveguide section 2 at a point substantially midway pf its length. Such a prior art window provides a relatively broad band structure typically having apassband of approximately 20% and being capable of passing relatively high power, as of MW peak power and KW average power.

When the prior art windowof FIG. 1 is employed as partially cut away, of a the output window in the typical microwave tube it is desired to utilize a waveguide transformer for connect ing the normal height waveguide 5 to the outputwaveguide of the microwave tube. The output waveguide of the microwave tube is shown at 11 inFlG. 2*and is normally of reduced height having a height h, as of 15 of its width, where the heigh h and the width w are shown inFlG. 1. The transformer is indicated at 12 in FIG. 2 and it may comprise a uniform taper in height of the guide or, as an alternative, one or more steps in the height of the guide, as shown in FIG. 2, for effecting an impedance match between the low height guide 11 and the nonnalheight guide 5. Such impedance transformers 12 utilize additional space and are relatively costly to manufacture.

Referring now to FIG. 3 there is shown a composite microwave window and waveguide transformer 15 incorporating features of the present invention. The structure of the composite microwave window and waveguide transformer 15 is substantially the same as that previously described with regard to FIG. 1 and 2 with the exception that the input waveguide 5, which is connected to the central waveguide section 2, is of reduced height. More particularly the input waveguide is of rectangular cross section having a height h approximately #5 of its width w. The coupling hole 7 in the end wall 4 is of rectangular cross section having a cross section of the same dimensions as the inside cross section of the low impedance input waveguide 5.

The microwave window member 9 is sealed across the center cylindrical waveguide section 2 at a point which is displaced from the midpoint toward the waveguide of reduced height, namely, waveguide 5. The window member 9 is displaced by an amount to produce a broadband match such that the composite microwave window with the displaced window member 9 forms a composite microwave window and impedance transformer. In a typical example, the center of the microwave window 9 is displaced from the midpoint of the length of cylindrical waveguide 2 by approximately 17% of the length of the waveguide 2. Waveguide 2 has a length l which is approximately 85% of k of a free space wavelength. The window 9 is dimensioned such that the distance d between the regions just inside the connecting waveguides 5 and 6, at the abrupt transitions 7 and 8, is approximately A of an electrical wavelength when taking into effect the loading produced by the window 9 and the fringing field regions at the abrupt transitions, such fringing field regions extending slightly into the adjacent waveguide sections 5 and 6. Alternatively, d could be n/2 electricial wavelengths where n is any odd integer.

In a typical example at s band, input waveguide 5 had a width of 2.84 inches and a height which was of the normal height, namely 0.892 inch. The output guide 6 had a width of 2.84 inches and a height of 1.34 inches. The cylindrical waveguide section 2 had a diameter of 9.03 centimeters and a length l of 3.85 centimeters. The window member 9 was made of alumina ceramic having a thickness of 0.36 centimeters.

The composite microwave window and waveguide transformer of the aforecited dimensions had a passband characteristic as shown in FIG. 4, wherein the passband was approximately 13% or 400 MHz between VSWR points of 1.1 centered at a frequency of approximately 2.8 GHz. The composite microwave window and transformer 15 passed 4 MW peak and greater than 8 KW average power. The output waveguide 6 and that portion of the central waveguide section 2 in gas communication therewith was pressurized with SP at a pressure of approximately 25 psi gauge. The input waveguide 5 was connected via flange 17 to the output waveguide of the microwave tube not shown, and the output waveguide 6 was connected to conventional standard size S- band waveguide via waveguide flange 18.

What is claimed is:

1. In a high frequency gas tight wave permeable window assembly dimensioned for passing wave energy therethrough over a certain passband of frequencies, a first tubular waveguide member having a first height, second and third tubular waveguide members having waveguide heights substantially less than the height of said first guide, centrally apertured end walls of said first waveguide member forming abrupt waveguide transitions for connecting said second and third waveguide members to said first waveguide member substantially at the ends of the said first waveguide member, a gas tight wave permeable window member disposed transversely of and within said first waveguide member, the thickness of said wave permeable window member being substantially less than A of an electrical wavelength at the center frequency of the passband of the wave permeable window assembly, and the length of said first waveguide member plus the fringing field regions extending into the adjacent second and third waveguides at said abrupt transitions, and with said wave permeable window in place, being approximately n/2 electrical wavelengths long at the center frequency of the certain passband of said window assembly, where n can have any odd integer value, said second waveguide member having a substantially uniform lower height than said third waveguide member, and said wave permeable window member being substantially displaced from the midlength position in said first waveguide toward the end thereof connecting to said second waveguide, whereby a relatively broadband composite microwave window and waveguide transformer is obtained.

2. The apparatus of claim 1 wherein said first waveguide is of circular cross section, said second and third waveguides are of rectangular cross section, and said apertures in said end walls of said first waveguide are each rectangular.

3. The apparatus of claim 2 wherein said second waveguide has a height approximately 9a of its width, and said third waveguide has a height which is approximately :6 its width.

4. The apparatusof claim 3 wherein said window is displaced by approximately 17% of the length of said first waveguide from the midlength position in said first waveguide.

5. The apparatus of claim 4 wherein the length of said first waveguide in between said end walls is approximately of a free space wavelength atthe center of the passband of the composite window and transformer.

II! i k t 

1. In a high frequency gas tight wave permeable window assembly dimensioned for passing wave energy therethrough over a certain passband of frequencies, a first tubular waveguide member having a first height, second and third tubular waveguide members having waveguide heights substantially less than the height of said first guide, centrally apertured end walls of said first waveguide member forming abrupt waveguide transitions for connecting said second and third waveguide members to said first waveguide member substantially at the eNds of the said first waveguide member, a gas tight wave permeable window member disposed transversely of and within said first waveguide member, the thickness of said wave permeable window member being substantially less than 1/2 of an electrical wavelength at the center frequency of the passband of the wave permeable window assembly, and the length of said first waveguide member plus the fringing field regions extending into the adjacent second and third waveguides at said abrupt transitions, and with said wave permeable window in place, being approximately n/2 electrical wavelengths long at the center frequency of the certain passband of said window assembly, where n can have any odd integer value, said second waveguide member having a substantially uniform lower height than said third waveguide member, and said wave permeable window member being substantially displaced from the midlength position in said first waveguide toward the end thereof connecting to said second waveguide, whereby a relatively broadband composite microwave window and waveguide transformer is obtained.
 2. The apparatus of claim 1 wherein said first waveguide is of circular cross section, said second and third waveguides are of rectangular cross section, and said apertures in said end walls of said first waveguide are each rectangular.
 3. The apparatus of claim 2 wherein said second waveguide has a height approximately 1/3 of its width, and said third waveguide has a height which is approximately 1/2 its width.
 4. The apparatus of claim 3 wherein said window is displaced by approximately 17% of the length of said first waveguide from the midlength position in said first waveguide.
 5. The apparatus of claim 4 wherein the length of said first waveguide in between said end walls is approximately 85% of a free space wavelength at the center of the passband of the composite window and transformer. 